The rate of rotation of a body about an axis may be determined by mounting an accelerometer on a moving frame, with the accelerometer's sensitive axis and the directon of motion of the frame both normal to the rate axis about which rotation is to be measured. For example, consider a set of orthogonal axes, X, Y and Z fixed in a body. Periodic movement of an accelerometer along the Y axis of the body with its sensitive axis aligned with the Z axis results in the accelerometer experiencing a Coriolis acceleration directed along the Z axis as the body rotates about the X axis. The acceleration or force acting on the accelerometer is proportional to a change in velocity of the body along the Z axis and its angular rate of rotation about the X axis. An output signal from the accelerometer thus includes a DC or slowly changing component representing the linear acceleration of the body along the Z axis, and a periodic component representing the Coriolis acceleration resulting from rotation of the body about the X axis.
In a similar manner, two accelerometers mounted with their sensitive axes aligned with the X and Y axes, and driven in a periodic motion along the Z and X axes, respectively, yield the magnitude of linear acceleration along the X and Y axes and the angular rate about the Y and Z axes. The processing of the output signals from the accelerometers to obtain angular rate and linear acceleration along the three orthogonal axes is described in commonly assigned U.S. Pat. Nos. 4,445,376, and 4,590,801.
In U.S. Pat. No. 4,510,802, a preferred embodiment of a rotational rate sensor is disclosed in which two accelerometers are mounted in a parallelogram structure with their sensitive axes parallel or antiparallel. The two accelerometers are vibrated back and forth in a direction substantially normal to their sensitive axes. An electromagnetic D'Arsonval coil mounted at one side of the parallelogram structure is energized with a periodically varying current, attracting a pole piece that is attached to the parallelogram structure. The varying magnetic attractive force of the coil causes the structure to vibrate, dithering the accelerometers back and forth. A signal processor connected to the pair of accelerometers combines their output signals, deriving both a rate signal and a linear acceleration signal.
Three such dithering parallelogram frame structures, wherein the sensitive axes of the associated accelerometer pairs are aligned along orthogonal axes, may be used to fully define the linear acceleration and angular rate of rotation experienced by a body. Although it is possible to build a relatively compact three axis rate sensor comprising three separate rate sensors of the above-described design, the total volume of three parallelogram structures may exceed the space available in certain applications. In addition, the cost of such a device may be prohibitive for a particular use.
Alternatives to the parallelogram frame structure for mounting a plurality of accelerometers are described in U.S. Pat. Nos. 4,445,376 and 4,522,062. The first of these patents discloses a triad of accelerometers, each mounted on a disk rotating about one of three orthogonal axes. The cost of the slip-rings required for this approach, and the electrical noise generated by the intermittency of a slip-ring contact limit the practical application of this design. In the other patent, a pair of accelerometers is mounted on a disk that dithers about a central axis. In one embodiment, the sensitive axes of the two accelerometers are aligned in parallel with the central dither axis and, in another embodiment, the two accelerometers are mounted on the disk back-to-back, with their sensitive axes radialy aligned normal to the dither axis. Because the pointing direction of their sensitive axes is continually changing as the accelerometers rotate back and forth, the signal produced by the pair of accelerometers has an undesirable cross-axis vibration sensitivity, i.e., vibrations in a direction nonaligned with the sensitive axis couple into the accelerometer's output signal as an error component. In addition, angular acceleration of the base on which the rotating disk is mounted tends to induce a natural resonance mode of the dithering disk, degrading the performance of the rate sensor. Neither of the rotating accelerometer techniques disclosed in these two patents substantially reduces the cost and size of a three-axis rate sensor, since a separate dither drive is required to determine rate about each orthogonal axis of interest.
It is thus an object of the present invention to provide a compact, lightweight and relatively low-cost three-axis rate sensor. A further object is to provide a rate sensor that is accurate and relatively insensitive to cross-axis vibrations and natural resonance mode excitation. These and other objects and advantages of the present invention as compared to the prior art rate sensors will be apparent from the attached drawings and the Description of the Preferred Embodiments that follows.